According to the popular bit holders (as shown in FIG. 1–3), such as those disclosed in U.S. Pat. No. 4,287,923 and 4,753,142, a conventional bit holder includes: a shank 1, on which a combination part 10 that can incorporate with hand tools was designed; and a C-shaped retaining groove 11 which would be buckled by a C-shaped buckle 110. And a reversion locking bulge 12 was provided. If necessary, just pressing the internal and external sleeve 15, 16 backwards would make the bit keep its extended position (see FIG. 3), and the screw held by the bit can be pulled out. Additionally, a plural locking bead holes 13 were designed for buckling round bead 130, and a combination groove 14, in which a flexible retaining groove 140 which would be buckled by a flexible buckle 141, was designed, and the flexible buckle 141 would be buckled within concaved pentagonal cover 170 of the bit 17 and combine the bit 17 with shank 1. In addition, an external sleeve 15 was designed to have a perforation 150 through which the shank 1 and a spring 151 and go; and an internal sleeve 16, whose rear holds one end of the spring 151, is equipped with a ring concaved groove 160, the ring concaved groove 160 can work with the action of the buckling round bead 130 and release the twisting force of bit timely (see FIG. 2).
However, because the extension length of bit cannot be adjusted as required when lock screw is up to a certain depth as shown in FIG. 1, the conventional bit holder cannot meet the locking requirement of various screws, for example, the pan screw's long head remained on the parts results in bad locking. Additionally, the bit 17 is fixed on the combination groove 14 of shank 1 with the flexible buckle 141 that can fix the bit well, but is hard to replace, and generally, a pincers is required for clipping it, which brings inconvenience to the user. For this purpose, the present invention can overcome the locking defects of the conventional bit holders and bring convenience to the user so as to meet the important requirement of innovation.